#include <iostream>
#include <iomanip>
#include <cmath>

using namespace std;

int main_0002()
{
    float a = 0.0;
    float b = 0.0;
    float c = 0.0;
    cin >> a >> b >> c;
    cout << fixed << setprecision(4) << sqrt((a + b + c) * (a + b - c) * (a + c - b) * (b + c - a)) / 4;
    return 0;
}
